Chapter 7: Bones

What are the seven important functions of bones?

1. Support for body and soft organs 2. Protection of brain, spinal cord, and vital organs 3. Movement as levers for muscle action 4. Mineral and growth factor storage (calcium and phosphorus) 5. Blood cell formation (hematopoiesis in red marrow) 6. Triglyceride (fat) storage in bone cavities 7. Hormone production (osteocalcin regulates insulin secretion, glucose levels, and metabolism)

How are bones classified according to shape?

1. Long bones: longer than they are wide (e.g., limb bones) 2. Short bones: cube-shaped (e.g., wrist and ankle bones) 3. Sesamoid bones: form within tendons (e.g., patella) 4. Flat bones: thin and slightly curved (e.g., sternum, scapulae, ribs, skull bones) 5. Irregular bones: complicated shapes (e.g., vertebrae, hip bones)

What types of tissues are found in bones?

Bones contain bone (osseous) tissue, nervous tissue, cartilage, fibrous connective tissue, muscle cells, and epithelial cells in blood vessels.

What are the three levels of structure in bones?

1. Gross 2. Microscopic 3. Chemical

What is compact bone?

Compact bone is the dense outer layer of every bone that appears smooth and solid.

What is spongy bone?

Spongy bone consists of a honeycomb of small structures called trabeculae, which are filled with red or yellow bone marrow.

What is the structure of short, irregular, and flat bones?

They consist of thin plates of spongy bone (diploe) covered by compact bone.

What are the main components of a typical long bone?

A typical long bone has a shaft (diaphysis), bone ends (epiphyses), and membranes.

What is the diaphysis of a long bone?

The diaphysis is the tubular shaft that forms the long axis of the bone, consisting of compact bone surrounding a central medullary cavity filled with yellow marrow in adults.

What are the epiphyses of a long bone?

The epiphyses are the ends of long bones, consisting of compact bone externally and spongy bone internally, with articular cartilage covering the joint surfaces.

What is the epiphyseal line?

The epiphyseal line is the remnant of the childhood epiphyseal plate where bone growth occurs, located between the diaphysis and epiphysis.

What is the periosteum?

The periosteum is a white, double-layered membrane covering the external surfaces of bones except at joints, consisting of a fibrous layer and an osteogenic layer.

What is the function of the fibrous layer of the periosteum?

The fibrous layer consists of dense irregular connective tissue and contains Sharpey's fibers that secure the periosteum to the bone matrix.

What is the endosteum?

The endosteum is a delicate connective tissue membrane covering the internal bone surface, covering trabeculae of spongy bone and lining canals in compact bone.

What is red bone marrow and where is it located?

Red bone marrow, or myeloid tissue, is located in the spongy bone and medullary cavity of long bones in children, forming blood cells.

Where is red bone marrow found in adults?

In adults, red bone marrow is limited to the flat bones of the skull, bodies of the vertebrae, head of the humerus, ribs, pelvis, and head of the femur.

What is yellow bone marrow?

Yellow bone marrow is formed from the degeneration of red bone marrow into a fatty substance and is found in adults.

What can cause the transformation of yellow bone marrow back into red bone marrow?

Severe anemia may cause the transformation of yellow bone marrow back into red bone marrow due to insufficient oxygen reaching cells of the body.

What are the five major cell types in bone tissue?

Osteogenic cells, osteoblasts, bone-lining cells, osteocytes, and osteoclasts.

What are osteogenic cells and their function?

Also called osteoprogenitor cells, they are mitotically active stem cells in the periosteum and endosteum that can differentiate into osteoblasts or bone-lining cells.

What do osteoblasts secrete?

Osteoblasts secrete unmineralized bone matrix called osteoid.

What is osteoid made up of?

Osteoid is made up of collagen and calcium-binding proteins.

What percentage of bone protein does collagen constitute?

Collagen makes up 90% of bone protein.

What are osteoclasts and their primary function?

Osteoclasts are giant, multinucleate cells that function in bone resorption, breaking down bone.

Where are osteoclasts located when active?

Osteoclasts are located in depressions called resorption bays.

What structural unit is referred to as an osteon?

An osteon, or Haversian system, is the structural unit of compact bone, consisting of an elongated cylinder that runs parallel to the long axis of the bone.

What are lamellae in the context of bone structure?

Lamellae are rings of bone matrix that contain collagen fibers running in different directions, providing strength and resistance to twisting.

What is the function of the central (Haversian) canal?

The central canal runs through the core of the osteon and contains blood vessels and nerve fibers.

What are perforating (Volkmann's) canals?

Perforating canals are lined with endosteum and occur at right angles to the central canal, connecting blood vessels and nerves of the periosteum, medullary cavity, and central canal.

What are lacunae?

Lacunae are small cavities that contain osteocytes.

What are canaliculi?

Canaliculi are hairlike canals that connect lacunae to each other and to the central canal.

What is the role of interstitial lamellae?

Interstitial lamellae fill gaps between forming osteons and are remnants of osteons cut by bone remodeling.

What are circumferential lamellae?

Circumferential lamellae are layers of lamellae that extend around the entire surface of the diaphysis, helping long bones resist twisting.

How does spongy bone differ from compact bone?

Spongy bone appears poorly organized but is structured along lines of stress, containing trabeculae that confer strength, while compact bone is organized into osteons.

What is ossification (osteogenesis)?

Ossification is the process of bone tissue formation, beginning in the second month of development.

What is endochondral ossification?

Endochondral ossification forms all bones inferior to the base of the skull, except clavicles, using hyaline cartilage models.

What is intramembranous ossification?

Intramembranous ossification begins within fibrous connective tissue membranes formed by mesenchymal cells, forming certain flat bones.

What are the four major steps involved in ossification?

1. Ossification centers are formed when mesenchymal cells cluster and become osteoblasts.

What role do capillaries in the endosteum play?

Capillaries in the endosteum supply nutrients to the bone.

What is the significance of osteocytes maintaining contact with each other?

Osteocytes maintain contact via cell projections with gap junctions, allowing communication and nutrient/waste exchange.

What condition may cause yellow bone marrow to transform back into red bone marrow?

Severe anemia due to insufficient oxygen reaching cells of the body.

What is osteoid made of?

Osteoid is made up of collagen and calcium-binding proteins.

What percentage of bone protein is collagen?

Collagen makes up 90% of bone protein.

What is the role of osteoclasts in bone tissue?

Osteoclasts are giant, multinucleate cells that function in bone resorption, breaking down bone.

What are resorption bays?

Depressions where active osteoclasts are located during bone resorption.

What is compact bone also known as?

Compact bone is also called lamellar bone.

What is an osteon?

The structural unit of compact bone, consisting of an elongated cylinder that runs parallel to the long axis of the bone.

What are lamellae in bone tissue?

Rings of bone matrix that contain collagen fibers running in different directions to withstand stress.

What is the function of interstitial lamellae?

They fill gaps between forming osteons and are remnants of osteons cut by bone remodeling.

What is the process of ossification?

Ossification (osteogenesis) is the process of bone tissue formation.

When does the formation of the bony skeleton begin?

In the second month of development.

What are the four major steps of intramembranous ossification?

1. Ossification centers form when mesenchymal cells cluster and become osteoblasts. 2. Osteoid is secreted. 3. Osteoid is calcified.

What is woven bone and how is it formed?

Woven bone is formed when osteoid is laid down around blood vessels, resulting in trabeculae.

What replaces woven bone during bone development?

Lamellar bone replaces woven bone, and red marrow appears.

What is required for interstitial growth in bones?

The presence of epiphyseal cartilage in the epiphyseal plate is required for interstitial growth.

How does the epiphyseal plate maintain its thickness?

The rate of cartilage growth on one side is balanced by bone replacement on the other.

What are the five zones of the epiphyseal plate?

1. Resting (quiescent) zone: relatively inactive cartilage on the epiphyseal side. 2. Proliferation (growth) zone: rapidly dividing cartilage on the diaphysis side. 3. Hypertrophic zone: older chondrocytes near diaphysis, with enlarged lacunae. 4. Calcification zone: cartilage matrix calcifies, chondrocytes die. 5. Ossification (osteogenic) zone: calcified cartilage spicules are eroded by osteoclasts and replaced with new bone.

What happens to the epiphyseal plate near the end of adolescence?

Chondroblasts divide less often, the epiphyseal plate thins, and is eventually replaced by bone, leading to closure.

At what ages does epiphyseal plate closure typically occur in females and males?

In females, around 18 years of age; in males, around 21 years of age.

What is appositional growth in bones?

Appositional growth is the process by which growing bones widen as they lengthen and can occur throughout life.

How do bones thicken in response to stress?

Bones thicken in response to increased stress from muscle activity or added weight.

What role do osteoblasts play in bone growth?

Osteoblasts beneath the periosteum secrete bone matrix on the external surface of the bone.

What is the function of osteoclasts in bone remodeling?

Osteoclasts remove bone on the endosteal surface and are responsible for bone resorption.

What is the typical recycling rate of bone mass?

About 5-7% of bone mass is recycled each week.

How often is spongy bone replaced?

Spongy bone is replaced approximately every 3-4 years.

How often is compact bone replaced?

Compact bone is replaced approximately every 10 years.

What does bone remodeling consist of?

Bone remodeling consists of both bone deposit and bone resorption.

Where does bone remodeling occur?

Bone remodeling occurs at the surfaces of both the periosteum and endosteum.

What are remodeling units in bone?

Remodeling units are packets of adjacent osteoblasts and osteoclasts that coordinate the remodeling process.

How do osteoclasts contribute to bone resorption?

Osteoclasts dig depressions or grooves in the bone matrix, secrete enzymes and protons that digest the matrix, and phagocytize demineralized matrix and dead osteocytes.

What happens to osteoclasts after resorption is complete?

Once resorption is complete, osteoclasts undergo apoptosis.

What activates osteoclasts?

Osteoclast activation involves parathyroid hormone (PTH) and immune T cell proteins.

What is the most important hormone in stimulating epiphyseal plate activity during infancy and childhood?

Growth hormone.

How does thyroid hormone affect growth hormone activity?

It modulates the activity of growth hormone, ensuring proper proportions.

What hormones promote adolescent growth spurts and induce epiphyseal plate closure?

Testosterone in males and estrogens in females.

What is the effect of excesses or deficits of hormones on skeletal growth?

They cause abnormal skeletal growth.

What is the role of parathyroid hormone (PTH) in calcium regulation?

PTH is produced in response to low blood calcium levels, stimulates osteoclasts to resorb bone, and raises blood calcium levels.

When does PTH secretion stop?

When homeostatic calcium levels are reached.

What hormone is produced in response to high blood calcium levels and what is its effect?

Calcitonin is produced, and its effects are negligible, but it can lower blood calcium levels temporarily at high pharmacological doses.

What role does calcitriol play in calcium absorption?

Intestinal absorption of calcium requires vitamin D (calcitriol).

What hormone released by adipose tissue may regulate bone density?

Leptin.

How does serotonin affect osteoblast activity?

Serotonin interferes with osteoblast activity and may inhibit bone turnover after a meal.

What does Wolf's law state about bones?

Bones grow or remodel in response to the demands placed on them.

What happens to bones when they are stressed?

They tend to bend, compressing one side and stretching the other.

Why can bones be hollow?

Because compression and tension cancel each other out in the center of the bone.

What triggers remodeling in bones when they are deformed?

Mechanical stress produces electrical signals.

What happens to the charges in compressed and stretched regions of bone?

They become oppositely charged.

How does mechanical stress influence bone remodeling?

It determines where remodeling occurs, while hormonal controls determine whether and when it occurs.

What underlies nearly every disease that affects the human skeleton?

Imbalances between bone deposit and bone resorption.

What is the significance of calcium in the body?

Calcium functions in nerve transmission, muscle contraction, blood coagulation, gland and nerve secretions, and cell division.

What percentage of the body's calcium is found in bone?

99% of 1200-1400 grams of calcium.

How does fluid flow within canaliculi affect bone remodeling?

Changes in fluid flow may stimulate remodeling.

What is the relationship between mechanical stress and bone density?

Mechanical stress can influence bone density by affecting the activity of osteoblasts and osteoclasts.

What is osteomalacia?

A bone disease where bones are poorly mineralized, resulting in soft, weak bones and pain upon bearing weight.

What causes rickets in children?

Rickets is caused by vitamin D deficiency or insufficient dietary calcium, leading to bowed legs and other bone deformities.

What characterizes osteoporosis?

Osteoporosis is characterized by bone resorption exceeding bone deposit, leading to a decline in bone mass, particularly affecting spongy bone in the spine and neck of the femur.

Who is most commonly affected by osteoporosis?

Aged, postmenopausal women are most commonly affected, with 30% of women aged 60-70 and 70% by age 80.

What role does estrogen play in bone density?

Estrogen helps maintain bone density, and its drop at menopause increases the risk of osteoporosis in women.

What are some additional risk factors for osteoporosis?

Insufficient exercise, poor diet in calcium and protein, smoking, genetics, hormone-related conditions, hyperthyroidism, diabetes mellitus, and alcohol consumption.

What are traditional treatments for osteoporosis?

Traditional treatments include calcium and vitamin D supplements, weight-bearing exercise, and hormone replacement therapy (HRT).